If you get the fact that he has more value than I don't understand why you don't get the rest of it.
If at all possible you play him where he has the most value.
If the O's get players that can provide more value than he can in left and first than he will provide more value at DH, in which case he should play DH.
I also don't think we've seen near enough to declare the kid a DH. The advantage in defense in left field at OPACY isn't that compelling an argument that we should run someone like Mullins out there. How many plays are the guys you mentioned going to make that Mountcastle won't?